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Investment Securities (Fair Value of Hedge Funds and Private Equity Funds) (Details) - USD ($) $ in Millions | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Hedge and private equity funds included in other long-term investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other long-term investments | $ 151.6 | $ 125.3
Unfunded Commitments | 131.0 | 58.1
Hedge funds | Hedge Fund, Long/ Short Equity Banks and Financial |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other long-term investments | 59.6 | 54.9
Unfunded Commitments | 0.0 | 0.0
Hedge funds | Hedge funds |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other long-term investments | 59.6 | 54.9
Unfunded Commitments | 0.0 | 0.0
Private equity security | Industrial |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other long-term investments | 49.6 | 43.3
Unfunded Commitments | 10.5 | 10.4
Private equity security | Aerospace Defense Government |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other long-term investments | 25.5 | 15.8
Unfunded Commitments | 34.9 | 12.9
Private equity security | Direct Lending |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other long-term investments | 10.6 | 7.1
Unfunded Commitments | 19.9 | 23.1
Private equity security | Financial Services Sector |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other long-term investments | 6.3 | 4.2
Unfunded Commitments | 15.7 | 11.7
Private equity security | Private equity security |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other long-term investments | 92.0 | 70.4
Unfunded Commitments | 131.0 | 58.1
Private equity security | Real estate |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other long-term investments | 0.0 | 0.0
Unfunded Commitments | $ 50.0 | $ 0.0

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- Definition The amount of unfunded commitments for which the entity is obligated, to those certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 6A -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=99377789&loc=SL6742756-110258 + Details
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